1. Field
The disclosed embodiments relate to substrate processing apparatus and, more particularly, to substrate processing apparatus with features to determine substrate alignment.
2. Brief Description of Related Developments
The manufacture of semiconductor devices, flat panel displays, and like products typically employ precision transport of substrates by specialized transport apparatus. Many of the involved manufacturing steps use a particular environment such as ultra-purified air or vacuum. The high cost of creating and maintaining such environments has led to constraints on substrate transport including narrow transport pathways, high precision substrate placement, and high speed of operation. Although substrate locations may be tracked via computer, actual locations may vary from model locations because of variations in placement within a substrate cassette, movement induced in a manufacturing process, small imperfections in system alignment and calibration, or other factors.
There are a number of conventional techniques for ascertaining the precise location of a substrate, so that positional errors may be corrected. One example of a conventional alignment system is described in U.S. Pat. No. 5,563,798. This document discusses calculating a wafer position from two or more discrete data points detected on the edge of the wafer as the wafer is transported by a transport robot. The position of the robot may then be adjusted to compensate for wafer misalignment. Similarly, U.S. Pat. No. 6,198,976 discusses calculating the center of a substrate relative to a reference point, using at least three discrete data points on the edge of the substrate. There are several problems with these types of conventional systems that rely on detection of discrete points. For one, the systems generally require multiple sensors to detect the discrete edge points. This complexity increases both and cost and the possibility for error from a defect in construction or calibration. Yet if only one sensor is used, the substrate must be repeatedly passed through the sensor so that the multiple edge points may be detected. The repeated passing of the substrate through the sensor increases the time required to complete an alignment process. Another problem with systems that detect discrete edge points is that they are subject to error due to variations in the wafer edge. These variations may throw off the location of one or more data points.
In other conventional techniques, prealigner devices may operate through rotating a substrate on a spindle and scanning the edge thereof with a sensor, to determine substrate position error or eccentricity. However, this typically requires execution of additional steps by a transport apparatus, which must place the substrate on the prealigner and then, after some duration, pick the substrate from the prealigner. These extra steps may slow down system operation. The exemplary embodiments described below may allow determination of substrate position while the substrate is in transit or, in other words, “on the fly”, and they overcome the problems of conventional positioning systems.